In the mid-1960s, wealthy socialite Jennifer Stirling suffers memory loss after a car crash, leaving her unable to recall much of her life before or connect with her husband Laurence Stirling. A cryptic letter addressed to "J" from "Boot" sparks intrigue, and Jennifer becomes determined to piece together what happened by following its clues.
In modern-day London, Ellie Haworth, freshly single, is tasked with writing about a recently deceased editor of her paper. She bypasses the archivist Rory McCallan to access the editor’s files and discovers a misfiled love letter addressed to "J" from "Boot." Moved by the ardor between the unknown lovers, Ellie becomes obsessed with uncovering their identities and retracing the beloved romance’s path.
Flashback to the past: a pre-crash Jennifer and her husband Laurence Stirling vacation on the French Riviera as foreign correspondent Anthony O'Hare arrives to interview Laurence about his business success. At a party, Laurence’s condescending attitude toward both Jennifer and Anthony sours the mood. Later, a inebriated Anthony vents about the night’s company, overheard by Jennifer.
The next morning, Anthony delivers a written apology he’s compelled to read aloud. He invites the Stirlings to join him for lunch the following day, but Laurence is called away on business, leaving Jennifer and Anthony to spend the summer together. They begin exchanging letters under the pennames "J" and "Boot" (or "B"), though neither acts on their attraction. Jennifer’s impulsive kiss is rebuffed, and Anthony, believing she rejected him, sends a letter proposing a meeting at Postman’s Park in London.
A whirlwind affair unfolds in which they steal moments together whenever Jennifer can discreetly be with Anthony. He eventually proposes the idea of fleeing to New York City together. Jennifer hesitates, fearing social exile and judicious scrutiny from family and friends. When Anthony promises to wait for her at the train station on the night of his departure, Jennifer rushes to meet him, only to be hurled into a car crash that leaves her with partial amnesia. Anthony departs for NYC, convinced that Jennifer has rejected him.
Six months after the crash, Laurence sequesters the final letter from Anthony, hoping Jennifer won’t remember the affair. Jennifer’s struggle to recover memories deepens as she uncovers more hidden love letters and discovers a postal box in her name that Laurence had closed. A confrontation reveals that Anthony may have died in the crash, leaving Jennifer to grapple with what was real.
Years pass, and Jennifer unexpectedly encounters Anthony, restoring memories of their time together. He again pleads with her to run away, but she resists because of their two-year-old daughter Esme Sterling. Yet she resolves to stay with him only for their child, vowing to leave if Laurence treats her poorly. Laurence retaliates with threats to ruin her reputation and win custody, forcing Jennifer to flee with their daughter to be with Anthony.
When she discovers Anthony has checked out of his hotel, she hurries to his workplace, only to learn he has already left. She reluctantly returns to Laurence and entrusts the bundle of letters to the editor to forward to Anthony if they ever hear from him.
In the present, Ellie and Rory grow closer as they uncover more of the love letters. After sharing a night together, Ellie steps back, realizing that Jennifer and Anthony are both alive. Hearing their regrets and heartbreak over a romance left unfinished, Ellie chooses to embrace romance with Rory instead of living with regrets. She persuades Anthony to compose one final letter to Jennifer, inviting her to meet him again at Postman’s Park. Ellie and Rory watch from afar as the two lovers reunite, their long-awaited moment finally realized.
